This application claims the priority of German Application No. 198 19 492.7, filed Apr. 30, 1998, the disclosure of which expressly incorporated by reference herein.
The invention relates to a method and device for measuring structures on a transparent substrate. The device includes an incident light-illuminating device, an imaging device, and a detector device for detecting imaged structures, as well as a measuring stage for holding the substrate. The measuring stage is displaceable in an interferometrically measurable fashion relative to an optical axis of the imaging device.
A measuring device of this type was described, for example, in the text of the paper entitled "Pattern Placement Metrology for Mask Making," presented by Dr. Carola Blasing at the Semicon meeting, Education Program, in Geneva, Switzerland on Mar. 31, 1998, the disclosure of which is incorporated by reference herein. The measuring device is set up in an environmental climate chamber in order to achieve measurement accuracy in the nanometer (nm) range. The position coordinates of various structures or features, such as pattern edges and the like, which are formed on substrates, such as photomasks and semiconductor wafers, are measured. The measuring device is mounted on a vibration-damped granite block. The masks and wafers are placed on the measuring stage by an automatic handling system.
The measuring stage moves by sliding via air bearings on the surface of the granite block. Plane mirrors arranged perpendicular to one another (orthogonal) are mounted at two sides of the measuring stage. A laser interferometer system determines the position of the measuring stage.
Using high-resolution apochromatically corrected microscope optics, the structures to be measured are illuminated and imaged with incident light in the near ultraviolet (UV) spectral range. A CCD camera serves as the detector for the imaged structures. Individual pixels form a detector array of the camera. Measurement signals are obtained from the individual pixels that are located within a defined measuring window. An intensity profile of the measured structure is derived from the measured signals by image processing, from which the edge position of the structure can be determined, for example.
The measuring device described above is preferably used for determining the edges of structures on masks and wafers for semiconductor manufacture. The structures are formed in layers which are applied to the mask surface. The surfaces have different degrees of reflectance. The edges (or flanks) of these layers have different steepnesses (slopes) because of the manufacturing process and are made with varying degrees of cleanliness. A TV autofocusing system is used in the measuring device to look for the setting with the sharpest contrast in order to optimize the image of the edge. The position of the edge measured at this setting relative to the optical axis of the imaging system, combined with the measured stage position, gives the position coordinates of the structural element in the machine's coordinate system. The position coordinates in the machine coordinate system can be converted into a mask coordinate system, following the previous alignment of the mask within the machine (measuring device), and compared with design data of the structural elements. In the case of linear structural elements for example, both edge positions can also be measured. The center line (or midpoint between the edges) can then be chosen as the position coordinate for this purpose. In cruciform (crossed) structures, the intersection of the center lines can be specified as the position.
The measured edge position depends on the physical quality of the edge itself and also on the optical measurement method employed, together with the quality of the imaging system. The relationships are described in the paper "Edge Measurement on Microstructures," W. Mirande, VDI Berichte, No. 1102 (1993), pages 137 et seq. If the position of the structure is defined by the center line to the two edges, the influences on the measured edge position generally have no effect on the measured position of the structure. The evaluation of the measurement results for a structural width measurement on the other hand can produce different results in different measuring devices.
In semiconductor manufacturing, the mask is illuminated with transmitted light in a stepper device and imaged onto the semiconductor wafer. Hence, there is interest in being able to determine the actual light-shading width of the structural element. For this purpose, special measuring microscopes have been developed in which the mask is illuminated by transmitted light and the width of the opaque structural image is measured exclusively. These known measuring devices however, do not provide for determining the position coordinates of the structural elements. These considerations likewise apply if transparent structural elements, rather than opaque structural elements, are to be measured in the mask surface.
The increasing quality control demands in mask manufacture require the manufacturer to check the design parameters of the mask structures, i.e. the structural elements, in terms of both their position and their effective projection width. Moreover, this checking process must be done within increasingly shorter processing times. The necessary cleanliness requirements also demand an increasing degree of care in the handling processes for placing the mask in the measuring device. With increasing mask size and increasing structural density, the value of the mask (for example its manufacturing cost) increases with each step in the process as well, so that careful handling in order to protect the masks from damage or destruction becomes a significantly important factor.
Hence, the goal of the present invention is to provide a method and measuring device with which both exact position determination and a reliable statement regarding the structural width of the structures on the substrate are possible without requiring knowledge of the geometric edge profiles of the substrate.
This and other goals are achieved in a measuring device according to the present invention in which the measuring stage is designed as an open frame with a ledge for receiving the substrate, and an illuminating device which uses transmitted light is provided beneath the measuring stage. The optical axis of the illuminating device is aligned with that of the incident light illuminating device. In a method suitable for achieving the stated goal, the substrate is illuminated, according to the invention, with either incident or transmitted light. Advantageous improvements follow from the features described herein.
The design of the measuring device which is known of itself according to the invention makes it possible to measure both the position coordinates and the structural widths of structures on substrates using incident and transmitted light illumination with only one handling process of the substrate in the same measuring device. This advantageously eliminates the set-up time which otherwise would be necessary for moving to a second measuring device and also minimizes the measuring time, thus speeding up the process and increasing the quality control. The elimination of an additional measuring device makes the inspection cheaper, simplifies the incorporation of an inspection measuring device into a processing line, and reduces the risk of the substrate being destroyed during handling. Direct comparison of the measured values using different types of illumination under evaluation conditions which are otherwise the same permits an expanded analysis of the mask manufacturing processes.
Surprisingly, it has been found that the measurement accuracy of the measuring device for measuring position coordinates can be retained even when the measuring stage is designed as a frame. The structural width can be measured with the same accuracy using transmitted light illumination as that of the position coordinates, and the structural width can be associated with a precisely determined position on the mask. The comparison of the measured values with the edge position using incident and transmitted light provides additional information about the design of the edges. In this manner, asymmetric edge profiles can be detected in particular, and the structural elements can be employed to correct the position coordinates.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.